Braking Action (BA) in words - Good, Medium and Poor only refer to Compacted Snow and Ice. See ICAO Annex 14 Green Pages and UK AIP Aerodrome Availability.
Also 'Unreliable' or 9 on the SNOWTAM could well be the machine is broke, so taking a 5kt crosswind limit may be a tad limiting.
For the certification basis for a/c look for either FAR Part 25 or in Europe CS 25.
Hope this helps
Sir George Cayley